Internal combustion engines generally fall into one of two categories. The two categories are (i) spark ignition and (ii) compression ignition. Spark ignition engines introduce a fuel/air mixture into combustion cylinders. The fuel/air mixture is then compressed in a compression stroke and ignited by a spark plug. Gasoline engines such as those found in automobiles are typically spark ignition engines. Compression ignition engines introduce or inject pressurized fuel into a combustion cylinder when the cylinder is in a condition referred to as “top dead center” (TDC). At this position, the cylinder is close to its maximum compression. At the pressure and temperature within the cylinder at TDC, the fuel is spontaneously ignited. Diesel engines are typically compression ignition engines.
Each of the above identified categories of engines have advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient while diesel engines are more efficient but produce more emissions.
The search for a combustion concept which marries the benefits of both spark ignition and compression ignition strategies has led to a concept called homogenous Charge Compression Ignition (HCCI). The HCCI combustion process has been studied for over two decades, and has shown significant promise as a potential technology for automotive engines that can improve on the efficiency and emissions capabilities of currently technologies. In this process, a homogeneous mixture of air, fuel and hot exhaust gases is compressed till auto-ignition occurs. Combustion is initiated not by a spark, but rather just through compression, and is governed by the in-cylinder temperature and mixture composition. A significant amount of hot exhaust gas from the previous cycle is usually trapped within the cylinder to enable this auto-ignition; however other methods for initiating HCCI have also been tested, including increasing the compression ratio and heating the intake air.
Another approach incorporates compression, but also relies upon spark ignition. This approach is referred to as Spark Assisted Compression Ignition (SACI). SACI combustion is mixed-mode form of combustion wherein a spark is used to initiate a propagating premixed flame that consumes a portion of the fuel and air charge to provide additional effective compression to the unburned mixture causing it to auto-ignite earlier than it would otherwise, thus combining characteristics of conventional spark ignition (SI) and compression ignition (CI) operation. The resulting heat release rate from SACI combustion therefore is faster than SI but slower than HCCI.
Various control algorithms have been developed to control timing of various operations in the combustion cycle in the various ignition approaches. In general, the control algorithms obtain inputs from various sensors and control various components in the engine system to optimize, for example, fuel efficiency, power, responsiveness, etc. One such control system is disclosed in U.S. Patent Publication No. 2013/0073173, published on Mar. 21, 2013, the entire contents of which are herein incorporated by reference.
Because the SACI approach is a mixed approach, however, the control algorithms which are used in other approaches are not optimized for characterizing the SACI auto-ignition event. By way of example, the duration of the auto-ignition combustion event is typically modeled as a function of the time or engine position at which the initiation of the combustion event is determined to occur. That is, the combustion duration is determined according to the following equation:ΔθAI=f(θAI)  (1)where “ΔθAI” is the auto-ignition combustion duration in time or crank angle, and
“θAI” is the time or engine position at combustion initiation. The functional dependence is typically assumed to be linear according to the following equation:ΔθAI=aθAI+b  (2)where a and b are parameters that can be fit according to operating characteristics. FIG. 1 depicts a chart 10 which plots the auto-ignition burn duration as a function of the start of auto-ignition over a range of different conditions and control inputs that might be used in SACI operation in reliance upon the foregoing equations. FIG. 1 shows that in SACI operation, the duration of the auto-ignition phase is effectively decoupled from the start of combustion and Equation (2) no longer holds.
In view of the foregoing, it would be advantageous to provide a more accurate approach for modeling the duration of the auto-ignition phase in SACI operation. A system including a method for more accurately predicting the duration of the auto-ignition combustion phase of an internal combustion engine when operated in a SACI mode would also be beneficial.